WASHINGTON—U.S. regulators slapped embattled Wells Fargo & Co. with new sanctions Tuesday, concluding the bank had failed to devise an adequate blueprint for avoiding a taxpayer bailout if it were on the brink of bankruptcy.

The rebuke over the bank’s so-called “living will” submission surprised executives and was another black eye for a bank that is still reeling from a scandal over its creation of bogus customer accounts. Even...